Intervertebral implant with conformable endplate

ABSTRACT

The invention is generally related to an intervertebral implant for replacing an intervertebral disc of the human spine. The intervertebral implant includes a first conformable endplate, the first conformable endplate being conformable to a boney vertebral endplate under an anatomical load, a second endplate and a core between the endplates, wherein the first conformable endplate partitions the core from the boney vertebral endplate, whereby the core does not contact the boney vertebral endplate. The invention is also directed to a method of replacing an intervertebral disc. The method includes removing at least a portion of an intervertebral disc to form an intervertebral disc space, implanting a first conformable endplate, into the intervertebral disc space and in contact with a first boney vertebral endplate, the first conformable endplate being conformable to the first boney vertebral endplate under an anatomical load; implanting a second endplate into the intervertebral disc space and in contact with a second boney vertebral endplate; and implanting a core between the first conformable endplate and the second endplate, wherein the first conformable endplate partitions the core from the first boney vertebral endplate, whereby the core does not contact the first boney vertebral endplate.

BACKGROUND OF THE INVENTION

[0001] A human intervertebral disc has several important functions, including functioning as a spacer, a shock absorber, and a motion unit. In particular, the disc maintains the separation distance between adjacent boney vertebral bodies. The separation distance allows motion to occur, with the cumulative effect of each spinal segment yielding the total range of motion of the spine in several directions. Proper spacing is important because it allows the intervertebral foramen to maintain its height, which allows the segmental nerve roots room to exit each spinal level without compression. Further, the disc allows the spine to compress and rebound when the spine is axially loaded during such activities as jumping and running. Importantly, it also resists the downward pull of gravity on the head and trunk during prolonged sitting and standing. Furthermore, the disc allows the spinal segment to flex, rotate, and bend to the side, all at the same time during a particular activity. This would be impossible if each spinal segment were locked into a single axis of motion.

[0002] An unhealthy disc may result in pain. One way a disc may become unhealthy is when the inner nucleus dehydrates. This results in a narrowing of the disc space and a bulging of the annular ligaments. With progressive nuclear dehydration, the annular fibers can crack and tear. Further, loss of normal soft tissue tension may allow for a partial dislocation of the joint, leading to bone spurs, foraminal narrowing, mechanical instability, and pain.

[0003] Lumbar disc disease can cause pain and other symptoms in at least two ways. First, if the annular fibers stretch or rupture, the nuclear material may bulge or herniate and compress neural tissues resulting in leg pain and weakness. This condition is often referred to as a pinched nerve, slipped disc, or herniated disc. This condition typically will cause sciatica or radiating leg pain, as a result of mechanical and/or chemical irritation against the nerve root. Although the overwhelming majority of patients with a herniated disc and sciatica heal without surgery, if surgery is indicated it is generally a decompressive removal of the portion of herniated disc material, such as a discectomy or microdiscectomy.

[0004] Second, mechanical dysfunction can cause disc degeneration and pain (e.g. degenerative disc disease). For example, the disc may be damaged as the result of some trauma that overloads the capacity of the disc to withstand increased forces passing through it, and inner or outer portions of the annular fibers may tear. These torn fibers may be the focus for inflammatory response when they are subjected to increased stress, and may cause pain directly, or through the compensatory protective spasm of the deep paraspinal muscles.

[0005] Traditionally, spinal fusion surgery has been the treatment of choice for individuals who have not found pain relief for chronic back pain through conservative treatment (such as physical therapy, medication, manual manipulation, etc), and have remained disabled from their occupation, from their activities of daily living, or simply from enjoying a relatively pain-free day-to-day existence. There have been significant advances in spinal fusion devices and surgical techniques. However, the procedures generally include shaping two adjacent boney vertebral endplates to conform to the endplates of the fusion device. The removal of bone from the endplates weakens the vertebral bodies and can lead to device stress shielding, bone remodeling, device subsidence, and device expulsion. Further, known endplates can lead to uneven distribution of loads across the vertebral bodies.

[0006] Known artificial discs offers several theoretical benefits over spinal fusion for chronic back pain, including pain reduction and a potential to avoid premature degeneration at adjacent levels of the spine by maintaining normal spinal motion. However, like spinal fusion surgery, the removal of bone from the vertebral endplates typically is necessary, thereby, weakening the vertebral bodies. Further, known endplates cause uneven distribution of loads across the vertebral bodies.

[0007] Therefore, a need exists for an intervertebral implant and a method replacing an artificial disc that overcomes or minimizes the above-referenced problems.

SUMMARY OF THE INVENTION

[0008] The invention is generally related to an intervertebral implant for replacing an intervertebral disc of the human spine. The intervertebral implant includes a first conformable endplate, the first conformable endplate being conformable to a boney vertebral endplate under an anatomical load, a second endplate and a core between the endplates, wherein the first conformable endplate partitions the core from the boney vertebral endplate, whereby the core does not contact the boney vertebral endplate. The implant can be an artificial disc or a fusion cage.

[0009] In one embodiment of the invention, the second endplate is conformable to a second boney vertebral endplate under an anatomical load. Further, the second endplate partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.

[0010] In one embodiment of the invention, at least one rigid plate can be disposed between at least one of the first and second endplates and the core, the rigid plate including a material which does not deform under the anatomical load.

[0011] In one embodiment of invention, the first and second endplate can include a textured surface that facilitates bone growth. The textured surface can include at least one member selected from the group consisting of porous beading, hydroxyapatite, and mesh. Optionally, the textured surface can be treated with an osteoinductive material. The osteoinductive material includes at least one member selected from the group consisting of a metallic, a polymeric, a ceramic, and a biologic material. The metallic material includes at least one member selected from the group consisting of titanium, cobalt-chromium, nitinol, and stainless steel. The polymeric material includes at least one member selected from the group consisting of polyethylene, polyester, polyurethane, silicone, and polycarbonate. The ceramic material includes at least one member selected from the group consisting of zirconia, alumina, hydroxyapatite, and tricalcium phosphate. The biologic material includes at least one member selected from the group consisting of collagen, bone morphogenic protein, a demineralized bone matrix, and a growth factor.

[0012] At least one protrusion element can be optionally coupled to a surface of at least one of the first and second endplates, the protrusion element being capable of penetrating a boney vertebral endplate, thereby securing a position of the first or second endplate to the boney vertebral endplate. The protrusion element includes at least one member selected from the group consisting of a keel, a spike, a tooth, a fin, and a peg.

[0013] In one embodiment of the invention, the conformable material includes at least one member selected from the group consisting of a metallic, a polymeric, and a biologic material.

[0014] In one embodiment of the invention, the core between the endplates, the core supporting boney vertebral endplates between which the conformable endplates have been placed and wherein the position of each conformable endplate is controlled at least in part by the boney vertebral endplate to which it is attached and is independent of the position of the other endplate. Optionally, the core can be a non-fluid or the core can include an osteoinductive rigid matrix which provides for spinal fusion.

[0015] In one embodiment of the invention, a kit includes at least two first conformable endplates. Each first conformable endplate is conformable to a boney vertebral endplate under an anatomical load. Each first conformable endplate has at least one dimension that is distinct from another conformable endplate of the kit. Each second endplate has at least one dimension that is distinct from another second endplate of the kit. A core is dimensioned for implantation between a first conformable endplate and a second endplate in an intervertebral space that has been prepared for placement of the first conformable endplate, the second endplate and the core. Upon implantation, the first conformable endplate partitions the core from a first boney vertebral endplate with which the first conformable endplate is in contact, whereby the core does not contact the first boney vertebral endplate.

[0016] In one embodiment of the invention, the second endplate is conformable to a second boney vertebral endplate under an anatomical load. Further, upon implantation of the second endplate and the core into an intervertebral space that has been prepared for placement of the first conformable endplate, the core and the second endplate, the second endplate partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.

[0017] In one embodiment of the invention, an intervertebral implant includes two conformable endplates. Each endplate includes a material that conforms to a boney vertebral endplate under an anatomical load and a core between the endplates. The core supports boney vertebral endplates between which the conformable endplates have been placed. The position of each conformable endplate is controlled at least in part by the boney vertebral endplate to which it is attached and is independent of the position of the other endplate.

[0018] The invention is also directed to a method of replacing an intervertebral disc. The method includes removing at least a portion of an intervertebral disc to form an intervertebral disc space, implanting a first conformable endplate into the intervertebral disc space and in contact with a first boney vertebral endplate. The first conformable endplate is conformable to the first boney vertebral endplate under an anatomical load. A second endplate is implanted into the intervertebral disc space and in contact with a second boney vertebral endplate. A core is implanted between the first conformable endplate and the second endplate, wherein the first conformable endplate partitions the core from the first boney vertebral endplate. The core does not contact the first boney vertebral endplate.

[0019] In one embodiment of the invention, the second endplate is conformable to the second vertebral endplate under an anatomical load. Further, the second endplate implanted partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.

[0020] In one embodiment of the invention, at least one rigid plate can be implanted between the core and at least one of the first conformable endplate and the second endplate.

[0021] The invention has many advantages. For example, the invention provides the boney vertebral bodies from succumbing to device stress shielding, bone remodeling, device subsidence, and device expulsion. Further, the invention also allows for even load distribution across the boney vertebral bodies.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 shows a perspective view of the lower region of the spine in which an intervertebral space has been prepared for implantation of the invention.

[0023]FIG. 2 shows a perspective view of one embodiment of the artificial implant of the invention being inserted into the prepared intervertebral space of the lumbar spine region of FIG. 1.

[0024]FIG. 3A shows an exploded perspective view of on embodiment of the implant of the present invention.

[0025]FIG. 3B shows an exploded perspective view of the implant of FIG. 3A with securing elements attached.

[0026]FIG. 3C shows an exploded perspective view of another embodiment of the present invention;

[0027]FIG. 3D shows an exploded perspective view of the implant of FIG. 3C with securing elements attached.

[0028]FIG. 4A shows a view of another embodiment of the present invention highlighting movement of spine in relation to the invention.

[0029]FIG. 4B shows another view of FIG. 4A.

[0030]FIG. 4C shows another view of FIG. 4A.

[0031]FIG. 5 shows a view of a prior art embodiment highlighting movement of spine about a pivot point.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The same number appearing in different drawings represent the same item. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0033] The present invention is related to a conformable implant intended to replace an intervertebral disc which has been removed due to disease, infection, deformity, or fracture, for example. FIG. 1 shows a perspective view of the lower region of a human spine 100. This region includes lumbar spine 120, sacral spine 130, and coccyx 140. The lumbar spine 120 is comprised of five (5) vertebrae L5, L4, L3, L2, and L1 (not shown). Intervertebral discs 150 link contiguous vertebra from C2 (not shown) to the sacral spine 130, wherein a single apostrophe (′) denotes a damaged disc, such as 150′.

[0034] Intervertebral disc 150 includes a gelatinous central portion called the nucleus pulposus (not shown) which is surrounded by an outer ligamentous ring called annulus fibrosus 160. The nucleus pulposus is composed of 80-90% water. The solid portion of the nucleus is Type II collagen and non-aggregated proteoglycans. Annulus fibrosus 160 hydraulically seals the nucleus pulposus, and allows intradiscal pressures to rise as the disc is loaded. Annulus fibrosus 160 has overlapping radial bands which allow torsional stresses to be distributed through the annulus under normal loading without rupture.

[0035] Annulus fibrosus 160 interacts with the nucleus pulposus. As the nucleus pulposus is pressurized, the annular fibers of the annulus fibrous prevent the nucleus from bulging or herniating. The gelatinous material directs the forces of axial loading outward, and the annular fibers help distribute that force without injury.

[0036] Although the following procedure is explained with reference to the lower spine, the procedure can be performed on any damaged disc of the spine. Further, the following procedure is described with reference to implants. However, it should be understood by one skilled in the art that an implant may be an artificial disc, a spinal fusion cage, or any other device known in the art.

[0037] According to one embodiment of the method of the invention, damaged disc 150′ is prepared to receive an implant of the invention by removing a window the width of the implant to be implanted from the annulus 160 of the damaged disc 150′. The nucleus pulposus of the disc 150′ is removed.

[0038] Referring to FIG. 2, once the damaged disc space is prepared, the surgeon chooses implant 200 of the invention from a kit of implants (not shown). The kit contains prefabricated and modular implants of various heights, shapes, and sizes. The surgeon inserts the chosen implant 200 into the intervertebral space 210 located between the superior endplate 220 of the inferior vertebra L5 and the inferior endplate 240 of the superior vertebra L4 (each vertebral body has a superior endplate and an inferior endplate).

[0039] The implant 200 may be inserted by hand or with an insertion instrument (not shown). If the implant 200 does not closely match the intervertebral space 210, the surgeon removes the implant 200 and chooses another implant 200 from the kit. This step is repeated until the surgeon determines the implant 200 which closely matches the intervertebral space 210.

[0040] The surgeon may then adjust the position of the implant 200 in the intervertebral space if needed. The implant can be adjusted in any direction within the intervertebral space 210. The implant 200 is now ready to be secured to vertebral endplates.

[0041] In one embodiment, either superior endplate 260 or inferior endplate 270 of the implant 200 conform to the defined contours (i.e. shapes) of superior or inferior endplates 220,240, respectively of vertebral bodies under an anatomical load. Preferably, both superior endplate 260 and inferior endplate 270 conform to boney vertebral endplates with which they are in contact. An “endplate” is defined as the portion of the implant that is in contact with a boney vertebral endplate when in use. If one endplate is not conformable to the boney vertebral endplate with which it is in contact, then that endplate can be a rigid material that is suitable for implantation, such as a rigid bio-compatible, metallic, polymeric or biologic material. In one such embodiment, the second endplate and the core are both rigid and, optionally, can be formed of the same material. In still another embodiment, the core and the second, rigid endplate are a single monolith. In the embodiment wherein the second endplate is rigid, the method of the invention can, optionally, include a step of preparing a portion of a second boney vertebral endplate for implantation of the second endplate, such as by grinding or cutting the second boney vertebral endplate. The anatomical load is the weight of the body above the resulting disc space, i.e., the weight of the body above disc space 210 in FIG. 2. In prior art techniques, the superior and inferior endplates of vertebral bodies were shaped to conform to the implants endplates.

[0042] The implant 200 can be further secured to the vertebral bodies by attaching at least one protrusion element (360 FIGS. 3B and 3D) to the superior and inferior endplates 260, 270 of the implant 200 to secure the implant 200 to vertebral endplates 220,240. The protrusion element 360 can be a keel, a spike, a tooth, a fin, or a peg.

[0043]FIGS. 3A and 3B show exploded views of a conformable implant 300 of an embodiment of the present invention and FIGS. 3C-3D show exploded views of a conformable implant 300′ of another embodiment of the present invention.

[0044] Each implant 300, 300′ has superior conformable endplate 310, inferior conformable endplate 320, and core 330 disposed between the superior endplate 310 and the inferior endplate 320. Each endplate 310,320 has an endplate surface 340 that is entirely conformable which allows for even load distribution across the boney vertebral bodies. Each endplate 310,320 also partitions the core from boney vertebral endplates contacting surface 340 of each endplate 310,320, whereby the core does not contact the boney vertebral endplates. However, in the embodiment of FIGS. 3C and 3D, a rigid plate 315 which does not deform under an anatomical load can be disposed between each endplate 310,320 and the core 330. It should be understood that a single rigid plate or multiple rigid plates can be used in any combination desired by the surgeon. For example, the surgeon may choose an implant 300′ with three rigid plates 315 disposed between the superior endplate 310 and the core 330 while having no rigid endplates 315 between the inferior endplate 320 and the core 330.

[0045] Each endplate 310,320 includes at least one member selected from the group consisting of a metallic, polymeric, or biologic material or any combination thereof which conforms to the boney vertebral endplate upon anatomical loading. Examples of these materials include, but are not limited to, titanium, cobalt-chromium, stainless steel, nitinol, polyethylene, polyester, polyurethane, silicone, polycarbonate, collagen, bone morphogenic proteins, demineralized bone matrices, growth factors, or other flexible materials which exceed the yield limit following loading which allows the endplate to conform. The thickness of the material varies depending upon the ductility of the material used, for example, titanium 6-4 can range between 0.0625 mm to 1 mm in thickness, whereas commercially pure titanium can range between 0.0625 mm to 6.35 inches in thickness.

[0046] Various methods known in the art can be employed singularly or in combination to help facilitate bone growth into the endplate. For example, each endplate 310,320 can include endplate surface 340 that is textured or roughened, whereby conformable endplate 310,320 bind to boney vertebral endplates upon boney ingrowth of the boney vertebral endplates into textured endplate surface 340 of each endplate 310,320. Examples of a textured or roughened endplate surface include porous beading, hydroxyapatite, and mesh. Further, endplate surface 340 of each endplate 310,320 can be coated with an osteoinductive material. Osteoinductive materials can be metallic, polymeric, ceramic, or biologic materials or any combination thereof. Examples of osteoinductive materials include, but are not limited to, titanium, cobalt-chromium, stainless steel, nitinol, polyethylene, polyester, polyurethane, silicone, polycarbonate, zirconia, alumina, hydroxyapatite, tricalcium phosphate, collagen, bone morphogenic proteins, demineralized bone matrices, growth factors or other materials known to facilitate bone growth.

[0047] Protrusion elements 360 can also be attached to the endplate surfaces 340 to provide against disc expulsion. Examples of protrusion elements include keels, spikes, teeth, fins, and pegs.

[0048] The core 330 of the implant 300,300′ can provide relative movement of the endplates 310,320 about the spine, such as a core in an artificial disc. An example of one such core is described in U.S. Pat. No. 5,401,269, and another example is described in U.S. Provisional Application No. 60/391,845, filed Jun. 27, 2002, the entire teachings of which are incorporated herein by reference. Alternatively, as is the case with a fusion cage, the core 330 of the implant 300,300′ can be made from an osteoinductive rigid matrix or cage with struts that are inter-packed with bone to provide short term rigidity and provide for long term ingrowth.

[0049] Referring to FIGS. 4A-4C, in another embodiment of the invention the implant 400 does not have a fixed pivot point within core 430. Each endplate 410,420 of implant 400 moves independent of each other, that is, each endplate 410,420 moves relative to its adjacent boney vertebra 450,460. The ability for implant 400 not to have a fixed pivot point allows the implant mimic a normal intervertebral disc of the spine. In contrast, prior art implants 500 as shown in FIG. 5 and described in more detail in U.S. Patent Publication 2003/0069642, the entire teachings of which are incorporated herein by reference, have a fixed pivot point 540 at the center of core 530 which does not allow for independent movement of endplates 510,520 relative to its adjacent boney vertebra 550,560.

[0050] Equivalents

[0051] While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. An intervertebral implant, comprising: (a) a first conformable endplate, the first conformable endplate being conformable to a boney vertebral endplate under an anatomical load; (b) a second endplate; and (c) a core between the endplates, wherein the first conformable endplate partitions the core from the boney vertebral endplate, whereby the core does not contact the boney vertebral endplate.
 2. The intervertebral implant of claim 1, wherein the second endplate is conformable to a second boney vertebral endplate under an anatomical load.
 3. The intervertebral implant of claim 2, wherein the second endplate partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.
 4. The intervertebral implant of claim 1, further including at least one rigid plate disposed between at least one of the first and second endplates and the core, the rigid plate including a material which does not deform under the anatomical load.
 5. The intervertebral implant of claim 1, wherein both the first and second endplate includes a textured surface that facilitates bone growth.
 6. The intervertebral implant of claim 5, wherein the textured surface includes at least one member selected from the group consisting of porous beading, hydroxyapatite, and mesh.
 7. The intervertebral implant of claim 5, wherein the textured surface is treated with an osteoinductive material.
 8. The intervertebral implant of claim 7, wherein the osteoinductive material includes at least one member selected from the group consisting of a metallic, a polymeric, a ceramic, and a biologic material.
 9. The intervertebral implant of claim 8, wherein the metallic material includes at least one member selected from the group consisting of titanium, cobalt-chromium, nitinol, and stainless steel.
 10. The intervertebral implant of claim 8, wherein the polymeric material includes at least one member selected from the group consisting of polyethylene, polyester, polyurethane, silicone, and polycarbonate.
 11. The intervertebral implant of claim 8, wherein the ceramic material includes at least one member selected from the group consisting of zirconia, alumina, hydroxyapatite, and tricalcium phosphate.
 12. The intervertebral implant of claim 8, wherein the biologic material includes at least one member selected from the group consisting of collagen, bone morphogenic protein, a demineralized bone matrix, and a growth factor.
 13. The intervertebral implant of claim 1, further including at least one protrusion element coupled to a surface of at least one of the first and second endplates, the protrusion element being capable of penetrating a boney vertebral endplate, thereby securing a position of the first or second endplate to the boney vertebral endplate.
 14. The intervertebral implant of claim 13, wherein the protrusion element includes at least one member selected from the group consisting of a keel, a spike, a tooth, a fin, and a peg.
 15. The intervertebral implant of claim 1, wherein the conformable material includes at least one member selected from the group consisting of a metallic, a polymeric, and a biologic material.
 16. The intervertebral implant of claim 1, wherein the core between the endplates, the core supporting boney vertebral endplates between which the conformable endplates have been placed and wherein the position of each conformable endplate is controlled at least in part by the boney vertebral endplate to which it is attached and is independent of the position of the other endplate.
 17. The intervertebral implant of claim 16, wherein the core is non-fluid.
 18. The intervertebral implant of claim 1, wherein the core includes an osteoinductive rigid matrix which provides for spinal fusion.
 19. The intervertebral implant of claim 1, wherein the implant is an artificial disc.
 20. The intervertebral implant of claim 1, wherein the implant is a fusion cage.
 21. A kit, comprising: (a) at least two first conformable endplates, each first conformable endplate being conformable to a boney vertebral endplate under an anatomical load, each first conformable endplate having at least one dimension that is distinct from another conformable endplate of the kit; (b) at least two second endplates, each second endplate having at least one dimension that is distinct from another second endplate of the kit; and (c) at least one core, the core being dimensioned for implantation between a first conformable endplate and a second endplate in an intervertebral space that has been prepared for placement of the first conformable endplate, the second endplate and the core, wherein, upon implantation, the first conformable endplate partitions the core from a first boney vertebral endplate with which the first conformable endplate is in contact, whereby the core does not contact the first boney vertebral endplate.
 22. The kit of claim 21, wherein the second endplate is conformable to a second boney vertebral endplate under an anatomical load.
 23. The kit of claim 22, wherein the second endplate, upon implantation of the second endplate and the core into an intervertebral space that has been prepared for placement of the first conformable endplate, the core and the second endplate, partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.
 24. An intervertebral implant, comprising: (a) two conformable endplates, each endplate includes a material that conforms to a boney vertebral endplate under an anatomical load; and (b) a core between the endplates, the core supporting boney vertebral endplates between which the conformable endplates have been placed and wherein the position of each conformable endplate is controlled at least in part by the boney vertebral endplate to which it is attached and is independent of the position of the other endplate.
 25. A method of replacing an intervertebral disc, comprising the steps of: (a) removing at least a portion of an intervertebral disc to form an intervertebral disc space; (b) implanting a first conformable endplate, into the intervertebral disc space and in contact with a first boney vertebral endplate, the first conformable endplate being conformable to the first boney vertebral endplate under an anatomical load; (c) implanting a second endplate into the intervertebral disc space and in contact with a second boney vertebral endplate; and (d) implanting a core between the first conformable endplate and the second endplate, wherein the first conformable endplate partitions the core from the first boney vertebral endplate, whereby the core does not contact the first boney vertebral endplate.
 26. The method of claim 25, wherein the second endplate is conformable to the second vertebral endplate under an anatomical load.
 27. The method of claim 26, wherein the second endplate implanted partitions the core from the second boney vertebral endplate, whereby the core does not contact the second boney vertebral endplate.
 28. The method of claim 27, further including the step of implanting at least one rigid plate between the core and at least one of the first conformable endplate and the second endplate. 